UV light having a wavelength of 400 nm or less is divided into various bands based on the wavelength according to a rule of ISO 21348. 98% or more of the UV light reaching the surface of the earth by sunlight is UV light of UV-A region. The UV-A light has a wavelength of 315 nm to 400 nm. The UV-A light has effects on the human skin, and may cause, for example, a skin blackening phenomenon or skin aging. UV light from sunlight having a wavelength of 280 nm to 315 nm is defined as UV light of UV-B region (hereinafter, the UV light may also be referred to as “UV-B light”). About 2% of the UV light reaching the surface of the earth from the sunlight is the UV-B light. The UV-B light may have serious effects on the human body, causing, for example, skin cancer, a cataract, or a red spot phenomenon. Most of the UV-B light is absorbed in the ozone layer. However, the amount of UV-B light reaching the surface of the earth and the UV-B light arriving have increased due to the recent depletion of the ozone layer, thereby raising serious environmental concerns. The UV light of UV-C region (hereinafter, the UV light may also be referred to as “UV-C light) has a wavelength in the range of 100 nm-280 nm. Most of the UV-C light is absorbed in the atmosphere and hardly reaches the surface of the earth. However, the UV-C light reaches the surface of the earth in areas where the ozone layer has depleted, such as the southern hemisphere. The effects of UV light on the human body are variously quantified and a UV index is representative of the quantified ones and is defined as a value obtained by integrating the products of weighted values and UV intensities at respective wavelengths.
Due to the change of atmospheric environment and a cultural expansion of leisure sports, etc., exposure to UV light in everyday life has increased. The UV index keeps the public informed of the danger of exposure to UV light so as to prevent excessive exposure to UV light. When the excessive exposure to UV light is prevented, the public may maintain a healthy life, and an increase of social medical costs may be suppressed.